1. Field of the Invention
The present invention relates to a system and a method for polling in an Ethernet Passive Optical Network (EPON), and more particularly to a system and a method for polling in an EPON, in which the maximization of an availability ratio of traffic channels results from an efficient utilization of resources, including traffic channels and time slots, used for traffic transmission in an EPON-based system.
2. Description of the Prior Art
An explosive increase of Internet users and the rise of applications requiring huge bandwidth cause a bottleneck situation between a large-capacity Local Area Network (LAN) and a backbone network. EPON technology is a representative one of solutions which can relieve the bottleneck situation occurring by a congestion of service requirements as above.
The EPON provides a high-quality access service through low-priced Ethernet and passive optical devices. The EPON is applicable and practicable to Wavelength-Division Multiplexing-Passive Optical Network (WDM-PON) technology, and is thus recognized as one of technologies which are optimal for a Fiber-To-The-Home (FTTH)-based access network structure henceforth.
Generally, an EPON in the form of a tree-structure topology, forming a point-to-multipoint optical network includes an Optical Line Terminal (OLT), a passive optical splitter/coupler, and an ‘N’ number of Optical Network Units (ONUs). Multiple ONUs share one optical channel with one another, and then performs traffic transmission. In the EPON technology, two wavelengths are used so as to provide a cost-efficient and effective data transmission service to subscriber network users located at a relatively short distance called the “last-mile”, where one wavelength is used for downstream representing a direction from the OLT to the ONUs, and the other wavelength is employed for upstream expressing a direction opposite to downstream. Especially, during data upstream, the OLT uses a Time-Division Multiple Access (TDMA) scheme so as to avoid transmission collision, and then provides the multiple ONUs with bandwidth allocation service. In an EPON system, the OLT located in a central office manages a right to use an upstream channel, the distribution of time slots, and bandwidth allocation for the ONUs. It can be said that an efficient distribution and management of optical channel use time slots is an essential factor in determining the performance of the overall EPON system, and is thus linked directly with service quality for users.
In the EPON, two mechanisms are typically considered so as to efficiently accommodate bandwidth requirements. One scheme is polling scheduling for determining a time point and order required for bandwidth allocation. The other scheme is a Dynamic Bandwidth Allocation (DBA) for determining bandwidth to be allocated to each ONU based on request bandwidth by corresponding ONU. In the two aspects, research has been conducted on various bandwidth management methods. As representative research, Interleaved Polling with Adaptive Cycle Time (IPACT), Sliding Cycle Time (SLICT), Early Allocation (EA), etc., can be cited. They properly apply the polling scheduling to bandwidth allocation, thereby accepting bandwidth requirements. The existing studies on bandwidth allocation are based on three polling schedules, i.e., Poll and Stop Polling, Interleaving Polling, and Interleaved Polling with Stop.
The scheme of Interleaved Polling with Stop among them carries out computation on bandwidth allocation to all ONUs per polling cycle, and accordingly, can efficiently and adequately allocate bandwidth. Nevertheless, this scheme has the problem that optical channel idle time exists. Polling schemes carrying out various approaches for solving this problem have been proposed.
Also, the EA scheme is the most representative scheme among approaches for reducing an optical channel idle time, and raises an availability ratio of upstream channels. However, the EA scheme has problems in that it has significant channel idle time occurring in an environment where the amount of traffic is larger, and ultimately acts in the same manner as that of the Interleaved Polling with Stop. Accordingly, there has been a need for improving the problems.
The OLT uses a Multi-Point Control Protocol (MPCP) so as to allocate bandwidths to multiple ONUs connected thereto. The MPCP is a polling protocol for the permission and a report of traffic transmission, and is standardized by the IEEE Ethernet in the First Mile (EFM) Task Force. The MPCP includes two modes such as a general mode and an auto-discovery mode. The general mode includes a GATE message and a REPORT message, where the GATE message is used by the OLT so as to allocate bandwidths to the ONUs, and the REPORT message is used by each Optical Network Unit (ONU) in order to report its own buffer status to the OLT. The auto-discovery mode is used so as to check whether the ONUs are linked to one OLT. In the auto-discovery mode, the OLT and the ONUs collect information (e.g., a Round Trip Time (RTT), a Media Access Control (MAC) address, etc.) on the linked ONUs by using REGISTER, REGISTER_REQUEST, and REGISTER_ACK messages.
In the EPON, detecting a state of each ONU based on the MPCP, the OLT allocates bandwidths to the ONUs, and the multiple ONUs request the OLT to allocate bandwidth. An effective polling policy not only raises the utilization of optical channels corresponding to one of the biggest issues in EPON research, but also reduces the “last-mile” bottleneck by properly allocating bandwidth.
In the poll and stop polling scheme out of the above basic polling policies, an OLT sends a GATE message so as allocate bandwidth to one ONU, and then waits until data and a REPORT message are transmitted from the one ONU to it. The poll and stop polling scheme has an optical channel idle period having its length equal to the extent of that of RTT per GNU existing whenever the OLT allocates bandwidths to ONUs, and thus show the lowest network throughput among the three basic polling policies.
In order to reduce propagation delay caused by MPCP signaling, the Interleaved Polling scheme uses upstream and downstream in such a manner as to overlap each other. After an OLT sends a GATE message to an ONU_i, it sends another GATE message to an ONU_(i+1) before data and a REPORT message transmitted by the ONU_i arrive. Herein, upstream of the ONU_i and downstream of the ONU_(i+1) progress on one optical channel, for which two different wavelengths (e.g., 1550 nm for upstream, 1310 nm for downstream) in one optical line. The Interleaved Polling scheme can raise an availability ratio of optical channels, but determines bandwidth to be allocated based on REPORT information of one ONU, so that bandwidth cannot be optimally allocated in consideration of all ONUs.
So as to make up for disadvantages of the Interleaved Polling scheme, the Interleaved Polling with Stop scheme stops polling with respect to every cycle. In the Interleaved Polling with Stop, the OLT receives REPORT messages from all ONUs, and then computes bandwidths allocated to the ONUs with reference to request bandwidths required by the ONUs. Thereafter, the OLT sends a GATE massage to the relevant ONU in the order of arrival time of REPORT messages. However, in the Interleaved Polling with Stop scheme, after the OLT has received the REPORT messages from all the ONUs, computation time necessary to compute the allocated bandwidths and channel idle time approximating the RTT are generated every cycle.
So as to remove the disadvantages of the Interleaved Polling with Stop, the EA scheme has been suggested. In the EA scheme for reducing an idle of an optical cannel, an OLT determines an immediate allocation of bandwidth to a relevant ONU or a computation wait of bandwidth allocation based on an optional threshold value and request bandwidth reported by ONUs.
In the EA, the OLT predetermines a particular threshold value as a minimum guarantee bandwidth, when one ONU requires bandwidth smaller than the minimum guarantee bandwidth, does not wait until a computation time point necessary for bandwidth allocation, but immediately allocates bandwidth to the relevant ONU. When the OLT allocates bandwidth to the relevant ONU, it determines a transmission start time so that the relevant ONU can transmit upstream frames for computation time necessary to allocation bandwidth and channel idle time equivalent to RTT. The EA scheme has such an advantage that it can use a predetermined optical channel idle time. Nevertheless, when the level of traffic between nodes of the ONUs is high on the whole, the EA scheme does not immediately allocate bandwidth, and acts as in the Interleaved Polling with Stop scheme. Also, the EA scheme has excessive jitter and provides an unstable service, because order for bandwidth allocation (i.e., upstream permission) to the ONUs changes.
On this account, there has been a need for a scheme of implementing an EPON-base system in an even more stable manner, and simultaneously utilizing traffic channels in a much more efficient manner.